1. Field of the Invention
This invention relates generally to the prevention of deposits on equipment employed in a circulating water system such as heat exchangers and boilers. The deposit may be an inorganic scale (e.g. calcium carbonate in a boiler condenser) or it may be an organic biofilm such as algae or bacterial growth, entrained in a source of natural water. More specifically, this invention relates to a method and system for monitoring a circulating water system so as to provide an indication of the amount and of the need to add a counteracting agent to the circulating water system.
2. Background Art
Control of deposits on the walls of heat-exchanging water circulating equipment is important for many reasons. It is sufficient to call to mind that the deposit can cause turbulence, meaning both inefficient flow and an increase on pump demand, or it can reduce heat transfer which means reduced efficiency in a heat exchanger.
Consequently, deposit inhibitors are introduced into the water entering the circulating system. In the case of inorganics (CaCO.sub.3 , MgCO.sub.3, etc.), so-called water softeners (scale inhibitors) are used as the treatment, stopping formation of the scale by neutralizing the offending ions; in the case of biofilms, biocides are used to destroy kill and prevent growth of microorganisms. The term "deposit inhibitor" is here used in an eliminative sense: addition of chemical "product", added to the stream of water, either to eliminate (prevent) inorganic ions from participating in scale formation or to eliminate (preclude) biofilms by destruction or prevention of the organic body.
Because the chemistry of the deposits can vary so widely, the inhibitor may take other forms besides a water softener or a biocide. Acid or alkaline treatment may be employed to adjust pH. Indeed, the treatments (inhibitors) fall into major classifications: threshold inhibitors, dispersants, surfactants and crystal modifiers, as explained in The Nalco Water Handbook, Second Edition, McGraw-Hill, 1988. The present invention is not restricted to any particular deposit former or inhibitor feed product.
It is consequently customary to employ some means to determine if a deposit of a particular kind has formed to any appreciable extent on the walls of the equipment in contact with the moving or circulating body of water.
It has been proposed to determine the extent of deposit buildup by mimicking, as much as possible, the conditions in a system for which fouling is being monitored. The method includes the steps of withdrawing a continuous sample of the system water in a side stream tube or conduit, monitoring the flow rate of the sample, waiting for a steady state or equilibrium condition to be achieved, and measuring the heat transfer resistance across the wall thickness of the sampling tube. After a reference heat transfer rate, or the tube wall temperature is established, upset conditions are introduced into the system. These include changing the heat transfer rate and decreasing the flow rate of the sample water in the system, until fouling occurs. Fouling generally affects the operating conditions of the monitor. For example, if the heat transfer rate is being held constant, then fouling causes a decrease in temperature. Conversely, if the tube wall temperature is held constant, then an increase in heat transfer rate is required to maintain the constant temperature.
The method is described in an article entitled "Cooling Water Fouling Monitor Series Upsets, Evaluates Changes" appearing in Chemical Processing, April 1990, pp. 34-38. As is stated in the article, the known monitor and method is "very useful for tracking fouling rate variation resulting from system upsets and changes in treatment program and flow rate. Nevertheless, the method and monitor is most sensitive to dramatic upsets, such as when there is an acid overfeed, and is much less sensitive to gradual scaling occurring over long periods of time, often weeks or months. The method also requires close monitoring of the flow rate within the sidestream tube or conduit. Another drawback is that the described method requires compensation of upset conditions by manual control over the addition of product to check the effects of the upset conditions.
The known system has heretofore achieved certain benefits which, though useful, are subject to other disadvantages. The known system monitors that a deposit is in fact occurring or has already occurred. However, monitoring of a deposit which has already occurred may be unacceptable in certain applications. In most cases, a determination that an unwashed deposition has occurred on a pipe wall of a condenser or heat exchanger system comes too late to prevent deterioration of system operation. Reversing the effects of such deposition often requires system shutdown and either replacement of the system elements, such as pipes, or cleaning of the system by acidic agents. Either of these alternatives is undesirable from a standpoint of cost and efficiency and may also cause unwanted effects on the system hardware. For example, excessive or repeated cleaning also causes damage to the system elements.
Thus, what is necessary, is a monitoring and anti-fouling system which can anticipate formation of deposits in an operating system before they occur and to simultaneously counteract the conditions which can lead to fouling of the system well before deposits begin forming on the walls. An automatic compensating mechanism which is sensitive to both drastic upset conditions in the circulating water system and to deposits occurring over great periods of time is also desirable. Ideally, such an automatic compensating mechanism will automatically feed product into the circulating water system to compensate for both system upset conditions and gradual deterioration caused by continuous deposits at a time immediately upon sensing by the monitor of a predetermined set of characteristics indicative of either of the two upset condition or gradual scaling condition. Most preferably, the monitor and method sense these characteristics well before they occur in the circulating water system itself, and the automatic product feed mechanism is triggered before the undesirable conditions are permitted to cause damage to the system elements.